The present invention pertains generally to cluster tools and processes which use a cluster tool. More particularly, the present invention pertains to cluster tools used in various processes for applying a coating to a silicon wafer. The invention is particularly, but not exclusively, useful as a cluster tool which simultaneously removes water vapor from a chamber containing a wafer while establishing a vacuum on the chamber to shorten the coating process and maximize wafer movement through the cluster tool.
In order to more fully appreciate the cluster tool of the present invention, some discussion of the scientific principles behind a vacuum deposition process is helpful. In general, if two electrodes and the gas therebetween are at extreme low pressure, less than {fraction (1/10,000)} of atmospheric pressure, the resistance of the gas will break down and allow current flow between the two electrodes. This is called a xe2x80x9cglow dischargexe2x80x9d in the prior art.
Importantly, during the glow discharge process, the ionized gas atoms become attracted to and collide with the negative electrode (cathode), while the free electrons become attracted to and move towards the positive electrode (anode). Since electron mass is generally negligible relative to an atom, the electrons do not substantially affect the anode during collision. The mass of the ionized gas atoms, however, may be substantial relative to the mass of the atoms for the cathode material. Accordingly, when the ionized gas atoms collide with the cathode, the force of collision causes cathode material to be emitted. This phenomena is known as sputtering, or the removal of material from the cathode by ion bombardment during a glow discharge.
The material that sputters from the cathode coats the surrounding surfaces. If a sputtering device is designed in a certain manner, a uniform, quality coating of the sputtered material can be deposited onto a substrate. The deposition of sputter material to form such a coating is often referred to as a vacuum deposition process.
In the semiconductor industry, the vacuum process is used to coat silicon wafers, and certain devices that accomplish this result are known in the industry as cluster tools. To accomplish the vacuum process, however, the silicon wafer must be processed within a cluster tool capable of maintaining an extreme vacuum condition to ensure film purity. In addition to the vacuum issue, all water vapor molecules must be removed from the wafer to prevent contamination of the vacuum processes within the cluster tool, as water vapor is an undesirable process component that affects the quality of the finished wafer. Finally, outgoing wafers may be hot after undergoing a plurality of vacuum processes, and it is desirable to cool the wafers within the cluster tool, to reduce the risk of oxidation or corrosion.
Cluster tools typically consist of a loadlock chamber for receiving wafers and a plurality of vacuum processing chambers for processing the wafers. In a conventional cluster tool, the wafers are introduced to the loadlock in a 25-wafer cassette module. Typically, the entire cassette module of wafers is placed in the loadlock chamber, and the loadlock chamber (along with the cassette module) is pumped down to high vacuum. This is a time consuming process. If the volume in the loadlock vacuum chamber could be minimized, and the vacuum condition established is merely a xe2x80x9croughxe2x80x9d vacuum sufficient for further processing of the wafer within the cluster tool, the time required to establish a vacuum on the loadlock chamber will be reduced. Thus, the efficiency of the cluster tool would be increased.
Prior art cluster tools typically have a dedicated degassing chamber for degassing the wafer, or removing the entrained water vapor molecules from the wafer. In the prior art, once the cassette module is under xe2x80x9croughxe2x80x9d to mid-range vacuum conditions (10xe2x88x922 torr to 10xe2x88x923 torr, where 760 torr=1 Atm=atmospheric pressure), the wafers are successively placed in the dedicated degassing chamber for individual degassing prior to undergoing several vacuum processes. The dedicated degassing chamber is usually within the high vacuum body of the prior art cluster tool. What is desired is a cluster tool that degasses the wafers in the loadlock chamber under rough vacuum conditions. This eliminates the need for a dedicated processing position within the high vacuum body of the cluster tool.
Degassed water vapor is removed from the high vacuum body of the cluster tool by cryopumps. A cryopump typically consists of a refrigeration unit that maintains an extremely low temperature on an array of metallic plates. The array is positioned in communication with the chamber and the water vapor molecules therein, and the water molecules impinge on the plate and are frozen thereto. These cryopumps are expensive and difficult to maintain. What is desired is a cluster tool that eliminates the need for a dedicated cryopump for each degassing chamber. One way to eliminate a dedicated cryopump is to remove water vapor molecules from the wafer while the wafer is at rough vacuum in the loadlock chamber. This will reduce the amount of residual water vapor in the proximity of the high vacuum body of the cluster. If this can be accomplished, vacuum quality and vacuum process quality can be enhanced. Also, the required number of cryopumps for the cluster tool would decrease, which further decreases the maintenance requirements for the cluster tool and increases the overall reliability of the tool.
In the prior art, once each wafer from the cassette module has been degassed, the wafer is successively placed in a plurality of vacuum processing chambers and undergoes a plurality of vacuum processes. After completion of the processes, the wafer may be placed in a dedicated chamber for cooling. The wafer is then placed back into the cassette module, and another wafer is selected from the cassette to begin the processes discussed above. When all of the wafers in the cassette module have finished processing, the entire cassette module is removed from the loadlock chamber.
With the above configuration, however, the first wafer spends more time cooling after its process is completed than the final wafer. Conversely, the last wafer to be processed has spent more time xe2x80x9cdryingxe2x80x9d under vacuum before undergoing its vacuum processes. This can lead to a drift in measurable process quality in the wafers being loaded one at a time from the cassette module into the cluster tool for processing. This is known in the prior art as the xe2x80x9cfirst wafer effectxe2x80x9d. What is desired is a cluster tool with loadlock chambers that are capable of performing both degas and cooling functions during processing of the wafer, to standardize the amount of time each wafer spends at each step of the process and, thereby, to improve the consistency of the finished wafer quality.
U.S. Pat. No. 5,516,732, which issued to Flegal for an invention entitled xe2x80x9cWafer Processing Machine Vacuum Front End Method and Apparatusxe2x80x9d, discloses a station in which a preheating and degassing function are combined. Flegal, however, does not disclose a water pump positioned in fluid communication with the chamber for removal of the water vapor molecules at extreme vacuum. This is because the device, as disclosed by Flegal, is known as a xe2x80x9cfront end devicexe2x80x9d and must be connected to a cluster tool to perform a vacuum process. Accordingly, Flegal does not minimize the xe2x80x9cwater loadxe2x80x9d on the processing stations and does not minimize the number of cryopumps required to operate the cluster tool.
U.S. Pat. No. 5,902,088, which issued to Fairbaim et al for an invention entitled xe2x80x9cSingle Loadlock Chamber With Wafer Cooling Functionxe2x80x9d, discloses a device in which a plurality of trays for receiving wafers are mounted within a loadlock structure, with the loadlock structure also performing a cooling function. The loadlock structure disclosed by Fairbaim, however, is sized to hold twelve to fourteen wafers, and establishing a vacuum on this size of a loadlock chamber presents the same problems that the present invention overcomes. Further, like Flegal, Fairbaim does not disclose a water pump attached in fluid communication with the loadlock structure and also does not minimize the number of water pumps required for the overall cluster tool.
In view of the above, it is an object of the present invention to provide a cluster tool that increases throughput of silicon wafers by performing the degassing of the wafer simultaneously with establishing a vacuum for the loadlock chamber containing the wafer. This decreases the overall processing time and increases wafer throughput.
It is another object of the present invention to provide a cluster tool that performs the degassing and the cooling steps within the loadlock chamber, to eliminate the need for separate dedicated degassing and cooling chambers. It is another object of the present invention to provide a cluster tool with a loadlock chamber that is sized to process a single wafer at a time, thereby minimizing the required volume to be placed under vacuum and eliminating what is known as xe2x80x9cthe first wafer effectxe2x80x9d.
It is another object of the present invention to provide a cluster tool with a single high vacuum chamber which services two loadlocks. This configuration minimizes the number of cryopumps required for the cluster tool and maximizes the reliability of the cluster tool.
Each of the above features and objectives results in a cluster tool that is easier to operate and is more reliable. It is also simpler to manufacture at less cost.
A cluster tool for performing a vacuum process on a silicon wafer in accordance with the present invention includes at least two loadlock chambers and a high vacuum chamber that are provided within a metal block. The loadlock chamber extend completely through the block from the block top surface to the block bottom surface. The high vacuum chamber is in fluid communication with each loadlock chamber in the block. An isolation means for selectively isolating each loadlock chamber from the high vacuum chamber, preferably a slot valve, is placed in each respective path of fluid communication. A respective roughing pump is connected in fluid communication with each loadlock chamber for establishing an initial vacuum therein.
The cluster tool of the present invention further includes a high vacuum pump and a water pump. The high vacuum pump is mounted to the block in fluid communication with the high vacuum chamber. The water pump comprises a refrigeration unit and a cryoplate. The refrigeration unit is mounted to the block, while the cryoplate is attached in thermal communication with the refrigeration unit. More specifically, the cryoplate projects from the refrigeration unit through the block and into the high vacuum chamber.
The cryoplate is positioned within the high vacuum chamber so that it is between the loadlock chambers and is directly in front of each respective slot valve. When a loadlock chamber is placed in fluid communication with the high vacuum chamber, the high vacuum pump draws water vapor molecules from the loadlock chamber. The water vapor molecules impinge upon the cryoplate and freeze thereon. The orientation of the high vacuum pump and water pump between the loadlock chamber minimizes the number of water pumps and high vacuum pumps needed to establish and maintain a high vacuum on the loadlock chambers.
The loadlock chambers are sized and shaped to accommodate a single wafer in order to minimize the required volume which is to be pumped to high vacuum. Each loadlock chamber has a top wall, a bottom wall and side walls and includes a platen comprising a stem that merges into a bell portion, with the bell portion terminating at a flat bottom surface. The platen stem is slidingly positioned within an opening in the top wall and projects downwardly into the loadlock chamber so that the bell portion is completely within the loadlock chamber.
The loadlock chamber of the present invention also includes a holding means that is adapted to hold a silicon wafer, and a plurality of heat lamps. The holding means comprises a tray with a plurality of rails that extend downwardly from the top wall in a surrounding relationship with the bell portion of the platen. Each rail has an upper flange and a lower flange that project perpendicularly from the rail, thereby establishing an inverted F-shape for the rail. When the wafer is resting on the upper flanges, the wafer is immediately proximate the bell portion bottom surface. When the wafer is resting upon the lower flanges, the wafer is spaced-apart from the platen bottom surface.
The loadlock chamber of the present invention further includes a plurality of heat lamps that are mounted to the bottom wall of the loadlock chamber so that they illuminate upwardly into the loadlock chamber. When the wafer is resting on the lower flanges, the wafer is coextensive with a focal plane of uniform radiation intensity from the heat lamps. The heat lamps, tray and platen combine to establish a loadlock chamber which can perform both a cooling and a degassing function, thereby reducing the number of loadlock chambers required for the cluster tool.
For the method of the present invention, the wafer is placed in the loadlock chamber on the lower flanges, and an initial vacuum is established by the corresponding roughing pump. Simultaneously, the heat lamps are activated to radiate heat energy to the wafer, thereby causing the release of water vapor molecules from the wafer, thereby degassing the wafer. Next, the roughing pump is isolated from the loadlock chamber, the lamps are preferably turned off, and the loadlock chamber is aligned in fluid communication with the high vacuum chamber by opening the corresponding slot valve. A high vacuum pump produces a high vacuum in the high vacuum chamber and draws residual water vapor molecules from the loadlock chamber into the high vacuum chamber.
As the molecules are drawn into the high vacuum chamber, the molecules impinge on the cryoplate, as discussed above, and are frozen onto the cryoplate. In this manner, the silicon wafer undergoes an initial degassing and a high vacuum is quickly and easily established in the loadlock chamber. Thereafter, the wafer undergoes a plurality of manufacturing processes in a manner known in the art.